Let's start with the Files app, which is built into iPadOS (as long as you have iPadOS 11 or higher).
Enabling access to your cloud drives
Open the app, and you'll see it already links to iCloud Drive and any files already stored on your iPad. If you've installed the Google Drive or Dropbox app (or any other cloud drive you use) you'll want to to enable access to these, so tap on the 3 dot icon in the Browse menu, then choose Edit. Then you can toggle on access to any other drives. Tap Done when finished.
Sharing to Book Creator
I have a PDF to share, so I navigate to it first, then I long tap on it and choose Share. Then find the Book Creator icon and tap on it. I could also tap on the document to open it, and then tap the Share icon in the top right of the toolbar.
Add the document in Book Creator
Method 1: The Shared tab
In Book Creator, tap the + button and tap the Shared tab at the bottom of the menu. Here you'll find any items that have been shared with you from other apps. Simply tap on the item you want to add and it will be added to your page.
Method 2: Accessing the Files app in Book Creator
This method is actually even easier. Rather than sharing the content from the Files app to Book Creator, you can actually access the Files app from within Book Creator. To do this, tap the + button, ... More, Files, then from the Browse tab, find the file you want and add it to the page. You may need to tap the < Back button to find the location you're looking for.